In a power line carrier system or the like, with sine wave power signals being used as carrier wave, information signals are superposed over the power signals and sent out. At the receiving end, the information signals superposed over the power signals are separated with a filter or the like. In separating information signals from electric signals in this process, though the phase of separated signals varies with the characteristics of the filter, it poses no essential problem when the information signals are analog audio signals or the like.
Now, when a digital signal is sent by the power line carrier system, the phase of the digital signal will be shifted at the receiving end, and the waveform will be distorted on the time axis. Where pulse signals are to be sent by pulse code modulation (PCM), even if the waveform is distorted on the time axis, such distortion can be coped with by newly generating pulse signals at the receiving end.
However, where signals undergo spread spectrum multiplexing, no pulse signals can be newly generated at the receiving end unlike in a PCM system. This waveform distortion on the time axis in signal reception poses a grave problem in the spread spectrum system.
Furthermore, one of the known power line carrier systems is to superpose information signals over a carrier wave consisting of sine wave power signals on AC power lines, and this is used in relatively simple communication of control signals, such as on-off control of a motor or the like, and of audio and other analog signals or digital signals by a PCM system. Many of the signal superposing systems used therein have a coupling transformer between the superposing system and the AC power lines to superpose information signals over power signals. As a result, the phase and other aspects of the information signals are varied by a kind of filter formed by this coupling transformer and the AC power lines, and this is known to invite waveform distortion of the superposed information signals.
However, even though information signals are subjected to such waveform distortion, no essential trouble would occur to control signals or analog signals referred to above because of the character of their processing. Nor would digital signals in a PCM system be subject to so serious disturbance, thanks to signal regeneration or otherwise at the receiving end, that signal errors are uncorrectable. Therefore, they created no system problem.
On the other hand, the progress of multiple access technology in recent years has led to studies on various multiplexing systems, and particular interest is focused on CDMA by spread spectrum multiplexing, which permits multiplexing with a simpler configuration than frequency division multiplexing or time division multiplexing.
However, if it is attempted to communication such CDMA-multiplexed information signals by the above-mentioned power line carrier system, inverse spreading on the receiving side is made difficult by the waveform distortion of signals, which are thereby prevented from demodulation. In other words, as it is impossible, in a power line carrier system using power signals on AC power lines as carrier wave over which information signals are superposed, to superpose information signals of a desired waveform (e.g. a stepwise waveform as illustrated in FIG. 20) without distortion, it is impossible to accomplish communications by CDMA.
An object of the present invention, undertaken to solve the problem pointed out above, is to provide a transfer apparatus and a transfer method, using power lines, for separating waveform information signals, on the time axis without distorting their waveform, from synthesized signals formed by superposing any desired waveform signals over sine wave signals.
Another object of the invention is to provide a transfer apparatus and a transfer method, using power lines, permitting superposition of information signals of a desired waveform over power signals on AC power lines.